1. Field of the Invention
The invention relates to a radar-based method of measuring the level of a material in a container in which, by means of the antenna of a ranging device arranged above the highest level anticipated, microwaves are radiated downwards and reflected microwaves received and the received microwaves are evaluated for determining the echo waves reflected from the surface of the material, for measuring the transit time of the echo waves and for computing the distance of the material surface from the antenna of the ranging device from the measured transit time.
The level to be measured in the container is either the filling height, i.e. the height of the material surface above the bottom of the container, or the volume of the material. The method as indicated above directly produces the filling height as the difference between the known height of installation of the antenna above the bottom of the container and the measured distance of the material surface from the antenna. The material volume unambiguously relates to the filling height and thus results from the measured filling height.
In application of this method when the container is empty the microwaves are reflected from the bottom of the container instead of from the material surface. This results in no problem as long as the bottom of the container is flat. In this case the microwaves are reflected directly back to the antenna and their transit time corresponds to the distance between the antenna and the bottom of the container; this distance being termed the empty distance. Since the empty distance is the maximum distance anticipated, evaluation of the received microwaves in the ranging device merely needs to be done in the distance range extending as far as this empty distance.
When, however, this method is put to use in the case of a container having a curved container bottom, for example when a so-called dished bottom is employed, and when the container is empty, the microwaves are no longer reflected back to the antenna directly, due to the angle of incidence at the curved container bottom being other than 90.degree.. Instead, the microwaves attain the antenna after having been multiply reflected by the walls of the container. Accordingly, the transit time of the microwaves corresponds to a distance which is substantially greater than the empty distance. Evaluating the received microwaves only in the distance range extending as far as the empty distance would then result in no echo waves whatsoever being detected, i.e. the ranging device will thus not recognize the empty condition of the container, it instead indicating a fault condition.
2. Description of the Prior Art
In solving this problem hitherto for containers having a curved bottom, the evaluation of the received microwaves is permanently done in a distance range which is very much greater than the empty distance. The correct level is assigned to all transit times of echo waves measured between the shortest transit time corresponding to the maximum level (100%) and a very low level (for example 0.01%), whereas a level of 0% is assigned to all longer transit times.
One drawback of this solution employed hitherto is that a very large time and distance range always needs to be covered, i.e. also in the normal measuring operation with the container partly or completely filled.